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154 HYDRAULIC LIMES AND CEMENTS
at a point 150 mm. above the base ; at this height is fixed a perforated disc
with about 1050 holes 2 mm. in diameter per sq. dm. The funnel is prolonged
into a cylindrical tube 20 mm. in diameter and 100 mm. long, the lower extremity
being 50 mm. from the top of the cylinder beneath. The material to be tested
is poured on to the perforated disc in quantities of about 300 grams at a time,,
and is stirred with a wooden spatula 4.0 mm. wide to assist the passage through
the holes. During the filling care must be taken not to shake the apparatus..
The mean of three consecutive results is taken as the true value.
Some authorities advise determination of the litre-weight of the cement
when compressed by a definite number of blows (generally rooo).
5. Fineness of Grinding.-.....This is determined by sieving the. material,
through successive metal wire sieves and weighing the residue on each sieve..
Use is generally made of sieves of 900 4900 meshes per sq. cm., made of
Wires of diameter 0-15 and 0-05 mm. respectively. The test is ma.de on a
50 gram sample, the. results as percentages being obtained by adding two
results obtained with the same sieve. The sieving is conducted by hand,
and is regarded as complete, when 25 successive shakings do not cause more
than o-i gram of material to pass through the sieve. The results are
expressed by adding, for each sieve, the weights of the residues not able.
to pass through it.
6. Quantity of Water for Pasting (gauging). The quantity of
water required to give a, paste of normal
consistency (normal /HIS/C-) is measured a.s
follows: 4,00 grains of the material are
placed on a marble or xinc slab in the
form of a heap hollowed in the middle.
Into this is poured, in one lot, the whole of
the water considered necessary at 15 -20°,
the whole being mixed rapidly with a
trowel, for one minute with a rapid-setting'
cement or for three minutes with a slow-
setting cement or a hydraulic lime (in any
case for a. less time than is required for
setting to begin : x<v below),
With tin- same- trowel the
paste is immediately lilled into a
split, cylindrical metal or ebonite.
mould, 4 cm. high and iS cm. in
diameter on a smooth glass plate;
tlu1 surface of the cement is
smoothed with the trowel, care,
being taken to avoid compression. The 'idnitijcr ntmmcr is then allowed
to descend into the mortar by its own weight (joo grams). This rammer
consists of a small cylinder, i cm, in diameter, which replaces the needle
of the Vicat apparatus (Fig. 8). The consistency is normal when the
cylinder comes to rest with its bast; about (> mm, from the level of the glass
plate, as indieated on the scale. If the consistency is not normal, the test
is repeated with a different quantity of water. The proper amount of
water used is divided by 4 to give the quantity per 100 grains of material,.
removed by drawing a straight strip, held vertically, across the top of the